1. Field of the Invention
The present invention relates to a sheet conveying apparatus including a skew-feed correction member for correcting a skew-feed of a sheet, and to an image forming apparatus including the sheet conveying apparatus.
2. Description of the Related Art
An image forming apparatus such as a copier, a laser printer, an LED printer, a facsimile machine, a word processor, and a multifunctional printer (which is a combination of two of more thereof) that forms an image on a sheet by using an electrophotographic image forming method is previously known.
In the conventional image forming apparatus, while accuracy of an image forming position with respect to the sheet (hereinafter, referred to as an image forming position accuracy) is one of important image quality factors, the image forming position accuracy is deteriorated when the sheet is skew-fed (so becomes skewed during conveyance of the sheet). Accordingly, it has been known to correct a skew-feed of the sheet by a skew-feed correction member before the image is formed on the sheet by an image forming portion.
The strike method is a typical skew-feed correction method for the sheet. In the strike method the sheet is conveyed by a conveying roller provided upstream of a skew-feed correction roller, for example, in a conveyance direction so that a leading edge of the sheet can strike the skew-feed correction roller that is stopped. The leading edge of the sheet strikes the stopped skew-feed correction roller, whereby the leading edge of the sheet is aligned by the skew-feed correction roller (as the conveying roller continues to rotate). After that, the skew-feed correction roller starts to rotate, and the sheet is conveyed. With regard to the timing of when the skew-feed correction roller that has been stopped starts to rotate, generally the skew-feed correction roller starts to rotate after the elapse of a predetermined time (period) since the sheet conveyed by the conveying roller is detected by a sensor.
The applicant has recognized that as a sheet width (a sheet length in a direction orthogonal to the conveyance direction of the sheet) becomes smaller, a total abutment pressure between the sheet and the conveying roller is reduced. This is because a contact region between the conveying roller and the sheet is reduced. Hence the sheet conveying force when the conveying roller conveys the sheet is decreased as the sheet width becomes smaller. In other words, as the sheet width becomes smaller, a conveyance stability of the conveying roller is decreased (so the conveyance stability of a sheet of smaller width is decreased with respect to a sheet of greater width). The sheet conveyed by the conveying roller receives a sliding friction caused by a guide. Accordingly, a conveyance efficiency of the sheet and an amount of skew-feed thereof that can occur while the above-mentioned conveying roller is conveying the sheet varies depending on a difference in a width of the sheet to be conveyed. The conveyance efficiency refers to a ratio of an actually fed amount of the sheet by the conveying roller with respect to a conveyance amount by which the sheet is supposed to be conveyed by rotation of the conveying roller. The sheet conveying force is weaker and a slippage is more prone to occur between the sheet and the conveying roller for a sheet of smaller width. Therefore a skew-feed amount is increased, and the conveyance efficiency is decreased for a sheet of smaller width.
Consequently the skew-feed amount may be increased at the time of conveying a narrow sheet, and accordingly, skew-feed correction may not be sufficiently performed because a pair of skew-feed correction rollers may start to rotate before the leading edge of the sheet is aligned with the pair of skew-feed correction rollers. The applicant has internally considered delaying a time when the pair of skew-feed correction rollers start to rotate so that the leading edge of the narrow sheet can be sufficiently aligned with the pair of skew-feed correction rollers. However, in this case, when a wide sheet is conveyed, an amount by which the conveying roller feeds the wide sheet after a leading edge thereof strikes the pair of skew-feed correction rollers is increased. Accordingly, the sheet may be damaged between the conveying roller and the pair of skew-feed correction rollers. For example, an excessive loop may be formed on the sheet between the conveying roller and the pair of skew-feed correction rollers, the loop thus formed may excessively contacts a conveying guide, and the sheet may buckle. Such sheet buckling may generate noise.
Further, the applicant has recognized that when such a width size of the sheet is small, the conveyance efficiency of the sheet is decreased. Accordingly, it takes more time for the conveyed sheet to reach the skew-feed correction rollers after passing through the sensor. Hence, when control is performed so that the pair of skew-feed correction rollers can start to rotate after the sensor detects the sheet and the skew-feed correction is performed appropriately for the wide sheet, the pair of skew-feed correction rollers start to rotate before the leading edge of the narrow sheet is aligned with the pair of skew-feed correction rollers. Hence, the skew-feed correction may not be sufficiently performed for the narrow sheet. Meanwhile the applicant has also recognized that, when the control is performed so that the pair of skew-feed correction rollers can start to rotate after the sensor detects the sheet and the skew-feed correction is performed appropriately for the narrow sheet, there are the following problems. Specifically, when the wide sheet is conveyed, the amount by which the conveying roller feeds the wide sheet after the leading edge thereof strikes the pair of skew-feed correction rollers is increased, and accordingly, the sheet may be damaged between the conveying roller and the pair of skew-feed correction rollers. For example, an excessive loop may be formed in the sheet between the conveying roller and the pair of skew-feed correction rollers, whereby the sheet may buckle. Further, noise occurs following such buckling.
As described above, the applicant has recognized that, depending on the sheet width, there occur problems that a sufficient skew-feed correction effect may not be obtained, and that the sheet may be damaged at the time of the skew-feed correction.
In Japanese Patent Application Laid-Open No. H11-165915, a time when the roller struck by the sheet to be conveyed is stopped is shortened in an order of from JIS B4 through JIS A4 to a postcard size (so as the width decreases) to reduce a loop amount. The loop amount refers to an amount by which the roller arranged upstream of the roller struck by the sheet feeds the sheet during a period from a time when the leading edge of the sheet strikes the roller concerned to a time when the roller starts to rotate. However, the technology and technique disclosed in Japanese Patent Application Laid-Open No. H11-165915, do not address or solve the above-mentioned problems, which, as the applicant has recognized, are caused because the skew-feed is more prone to occur in the sheet and the conveyance efficiency of the sheet is also decreased when the width size of the sheet to be conveyed is reduced.
In recent years, in line with miniaturization of image forming apparatuses, there has been proposed an image forming apparatus having a configuration in which a sheet discharge roller that discharges the sheet to a sheet discharge tray is also used as a reversing roller used at the time of two-sided image formation (refer to Japanese Patent Application Laid-Open No. 2003-155146). If the sheet conveying force (a sheet discharging force) is too large, the sheet discharge roller (a reversing roller) sometimes disturbs stacking regularity of sheets previously stacked on the sheet discharge tray, and when the leading edge of the discharged sheet strikes a surface of the sheet discharge tray, the sheet sometimes curls. If nipping force to nip the sheet is increased in order to increase the sheet conveying force of the sheet discharge roller, then the image on the sheet may be damaged by the sheet discharge roller that contacts the sheet soon after the image is fixed. Because of the reasons as described above, the conveying force of the sheet discharge roller is sometimes set relatively small. Hence, when the sheet discharge roller conveys a narrow sheet (of narrow width), slippage is prone to occur between the sheet and the sheet discharge roller, and the above-mentioned phenomena that the skew-feed occurs to a large extent and the conveyance efficiency is decreased may become significant. Hence, in the case of a configuration in which the sheet discharge roller (the reversing roller) conveys the sheet so that the leading edge of the sheet strikes the skew-feed correction roller, the above-mentioned problem identified by the applicant and caused by the difference in width of the sheet at the time of the skew-feed correction for the sheet may become significant.